1. Field of the Invention
The present invention relates to duplexers including a transmission SAW filter device and a reception SAW filter device, and to modules that include such duplexers.
2. Description of the Related Art
A duplexer that includes a transmission SAW filter device and a reception SAW filter device has been proposed (see Japanese Unexamined Patent Application Publication No. 2010-178306 (paragraphs [0011] to [0018], FIG. 1, abstract, etc.), for example). In the conventional duplexer 500 shown in FIG. 9, a ladder-type transmission SAW filter device 504 that outputs, to a common terminal 503, an unbalanced transmission signal inputted from an unbalanced transmission terminal 502, and a longitudinally coupled resonator-type reception SAW filter device 507 that outputs an unbalanced reception signal, inputted to the common terminal 503, to balanced reception terminals 505 and 506 in a balanced state, are formed on one main surface of a piezoelectric substrate 501.
The transmission SAW filter device 504 includes serial arm resonators 504a to 504c connected in series between the transmission terminal 502 and the common terminal 503 and parallel arm resonators 504d and 504e connected at one end between the respective serial arm resonators 504a to 504c and connected at the other end to a ground terminal 508, and each of the resonators 504a to 504e is formed with reflectors disposed on both sides in the direction in which surface acoustic waves travel along interdigital electrodes. The reception SAW filter device 507 includes a longitudinally coupled resonator-type filter element 507b connected to the common terminal 503 via a resonator 507a that functions as a transporter and a longitudinally coupled resonator-type filter element 507c connected in a stage after the longitudinally coupled resonator-type filter element 507b.
The longitudinally coupled resonator-type filter element 507b is formed with three interdigital electrodes arranged in parallel in the direction in which surface acoustic waves travel and with reflectors disposed on both sides of the direction in which the interdigital electrodes are arranged. Of the three interdigital electrodes that form the longitudinally coupled resonator-type filter element 507b, one end of each interdigital electrode on either side is connected to the longitudinally coupled resonator-type filter element 507c in the later stage, and the other end is connected to a ground terminal 509 and grounded. One end of the interdigital electrode in the center is connected to the ground terminal 509 and grounded, and the common terminal 503 is connected to the other end via the resonator 507a and an unbalanced reception signal is inputted to that other end.
The longitudinally coupled resonator-type filter element 507c is formed with four interdigital electrodes arranged in parallel in the direction in which surface acoustic waves travel and with reflectors disposed on both sides of the direction in which the interdigital electrodes are arranged. Of the four interdigital electrodes that form the longitudinally coupled resonator-type filter element 507c, one end of each interdigital electrode on either side is connected to the ground terminal 509 and grounded, and the output of the longitudinally coupled resonator-type filter element 507b in the previous stage is inputted to the other ends of these interdigital electrodes. Of each interdigital electrode in the center, one end is connected to the reception terminals 505 and 506, the other end is connected to the ground terminal 509 and grounded, and a balanced reception signal is outputted from the reception terminals 505 and 506.
A shield electrode 510 that is connected to the ground terminal 509 and grounded is formed on the one main surface of the piezoelectric substrate 501 so as to surround the reception SAW filter device 507, in order to improve isolation characteristics between the transmission SAW filter device 504 and the reception SAW filter device 507. In the reception SAW filter device 507, the longitudinally coupled resonator-type filter elements 507b and 507c are configured so that balanced reception signals having almost the same amplitude but with phases that are shifted from each other by approximately 180° are outputted from the reception terminals 505 and 506, respectively.
However, the conventional duplexer 500 has a problem in that it is necessary to form the shield electrode 510 surrounding the reception SAW filter device 507 on the one main surface of the piezoelectric substrate 501 in order to improve the isolation characteristics between the transmission SAW filter device 504 and the reception SAW filter device 507, which increases the size of the duplexer 500.